1. Field of the Invention
The present invention relates to a magnetoresistive element, magnetic head and magnetic recording/reproducing apparatus.
2. Description of the Related Art
By virtue of the discovery of a magnetoresistive element that exhibits a giant magnetoresistive effect (GMR), the performance of magnetic devices, in particular, magnetic heads, has been significantly enhanced. In particular, since a spin-valve (SV) film has a structure that can be easily applied to magnetic devices, it has enormously contributed to technical development of magnetic discs.
A spin-valve film comprises two ferromagnetic layers and a non-magnetic layer interposed therebetween. One of the ferromagnetic layers, called a pinned layer, has a magnetization whose direction is pinned by, for example, an antiferromagnetic layer, while the other ferromagnetic layer, called a free layer, has a magnetization whose direction is made to respond to an external magnetic field. In this structure, a giant magnetoresistance change can be obtained in accordance with a change in the relative angle made by magnetization directions of the pinned and free layers. Theoretically, the spin-valve film enables efficient magnetic field detection if the magnetization direction of the free layer is made parallel to the track width direction when the external magnetic field is zero, and if the magnetoresistance change is generated when the magnetization direction of the free layer is changed in accordance with the external magnetic field (see U.S. Pat. No. 5,206,590).
Conventional spin-valve films are mainly of a current-in-plane (CIP) type in which a sense current is made to flow parallel to the film plane. On the other hand, spin-valve films of a current-perpendicular-to-plane (CPP) type are now being developed, in which a sense current is made to flow substantially perpendicular to the film plane, because they exhibit a much greater GMR effect than the CIP type. At the present stage, CPP spin-valve films are expected as a most-promising technique for realizing a magnetic recording/reproducing apparatus having an areal recording density of 200 Gbit/inch2 (Gbpsi) or more.
To achieve a high recording density, the size of the spin-valve film must inevitably be reduced. In other words, it is necessary to narrow the track width recorded on a medium for high recording density. Accordingly, it is also necessary to reduce the size of the free layer, as a sensing layer, of a spin-valve film. For instance, the track width of a spin-valve film is as narrow as about 100 nm if the areal recording density is 200 Gbit/inch2, about 50 nm for an areal recording density of 500 Gbit/inch2, and about 35 nm for an areal recording density of 1 Tbit/inch2. If the conventional CPP spin-valve film size is reduced in accordance with an areal recording density of 500 Gbit/inch2 or more, the following two serious problems may occur.
Firstly, a vortex domain may occur in the free layer. Assuming that the same sense current as in the conventional case is made to flow in the perpendicular direction, the smaller the free layer size, the higher the current density in the free layer. This causes a vortex domain in the free layer due to a current magnetic field. When a vortex domain occurs in the free layer, the magnetization direction of the free layer cannot be made parallel to the track width direction, resulting in unsatisfactory magnetic field detection. Such a vortex domain occurs when the current density is about 108 A/cm2 or more. If, for example, the recording density is 500 Gbit/inch2 and the sense current is 3 mA, the current density is 1.2×108 A/cm2 and accordingly a vortex domain occurs. In this case, it is not an effective countermeasure to reduce the sense current to about 1 mA so as to avoid the occurrence of the vortex domain. This is because the countermeasure involves reduction in the signal output voltage (i.e., current×resistance change).
Secondly, the influence of a spin transfer torque phenomenon may be serious (see, for example, Journal of Magnetism and Magnetic Materials 159 (1996), L1-L7). Assume that the size (one side) of an element having two magnetic layers and a non-magnetic layer interposed therebetween is set to 100 nm or less, and a current with a current density of 107 to 108 A/cm2 is made to flow through the element in the perpendicular direction. In this case, a phenomenon is observed in which a spin torque of one magnetic layer is transferred to the other magnetic layer, thereby changing the magnetization direction of the other magnetic layer. The occurrence of such a spin transfer torque phenomenon in the CPP spin-valve film means that, even if the external magnetic field from a medium is zero, the sense current changes the magnetization direction of the free layer. In other words, the phenomenon makes it difficult to realize the operating principle of the spin valve that the magnetization direction of the free layer is changed by the medium magnetic field to thereby detect a magnetoresistance change. In a CPP spin-valve film corresponding to a recording density of 500 Gbit/inch2 or more, the element size and current density are significantly influenced by the spin transfer torque phenomenon. Therefore, the operation of the CPP spin-valve film is inhibited by the phenomenon.
As stated above, in a CPP spin-valve structure of a small element size, two problems, i.e., the vortex domain problem and spin transfer torque problem, may well occur, which makes it difficult to realize a magnetic recording/reproducing apparatus having a recording density of 500 Gbit/inch2 or more.